1. Field of the Invention
The invention relates to mixtures of a polyisocyanate and a polyol which are storage stable at room temperature and are heat curable wherein the polyisocyanate, in the form of discreet particles which are deactivated on their surface, is dispersed in the polyol. The mixtures are used for coating metals and are particularly useful for undercoating motor vehicles.
2. Description of the Prior Art
The preparation of mixtures containing polyisocyanates and binders, which contain groups which are reactive with isocyanate groups, and their use is known.
Great difficulties, however, are incurred because the isocyanate group reacts with the binder, for instance, polyols or polyamines, at room temperature. Thus storage-stable combinations cannot be formulated.
In order to avoid this difficulty, two component systems are frequently used. The polyisocyanate and binder are stored in separate containers and are mixed immediately prior to application whereupon the reaction will then take place either spontaneously or by using heat and/or a catalyst. An example of such a system is described in U.S. Pat. No. 4,029,626. The drawbacks of this process lie in the need for the separate preparation and storage of the two components, the difficulties in the accurate metering and effective mixing prior to application, and in the danger of plugging the mixing and application equipment as a result of an early reaction.
Another familiar method for the manufacture of storage-stable polyisocyanate masses is based on non-crosslinked prepolymers which contain little free isocyanate and where moisture in the air serves as a crosslinking agent. This method is described, for instance, in French Pat. No. 1,237,936. Some of the problems with this method are that the curing from the surface towards the depth of the coating progresses very slowly, the final properties are reached only after weeks or months, and no curing takes place, for instance, between large areas or in hollow spaces.
Stable systems may also be formulated when the polyisocyanates are initially reacted with a monofunctional capping agent. The resultant adducts are referred to as capped or blocked isocyanates if they are less thermally stable than the products which are ultimately formed by the crosslinking reaction. Under the effects of heat, the capping agent is separated and the polyisocyanate enters into the more thermally stable bond which is accompanied by a crosslinking process. This principle is described, for instance, in the Vieweg/Hoechtlen, Plastics Handbook, vol. VII, page 11 ff, "Polyurethanes" (Carl Hanser Publishers, Munich, 1966), and is also the basis for a series of patents, for instance, German application No. 2,640,295, German application No. 2,612,638, German application No. 2,639,491 and/or European Patent application No. 0 000 60. Technical and economic drawbacks of such formulations result because of the separated capping agent which, if it remains in the crosslinked mass, changes the properties of this mass or, if it evaporates, must be reclaimed or must be removed from the air.
In German application No. 1,570,548 and German published application No. 2,842,805, polyisocyanates melting at above 100.degree. C. or at above 130.degree. C. are described which result in storage-stable combinations when reacted with certain polyols. The products can be cured and used as coatings for textiles and/or textile hoses. The technical drawback of these products is that they depend upon a very narrow selection of specially structured polyols in order to achieve good storage stability and are, therefore, not at all suited for a number of applications.
In view of the mixtures and processes described, there is a need to develop storage-stable, heat curable liquid to paste-like mixtures of polyisocyanate and polyol which do not have the above-referenced drawbacks.